Valence band offset of 2.3 ± 0.4 eV at strained NiO/MgO heterojunction is determined from photoelectronspectroscopy (PES) measurements. The determined value of valence band offset is larger than that is pre-dicted from first principle calculations, but is in corroboration with that obtained from band transitivityrule. Our PES result indicates a larger value of the valence band offset at strained NiO/MgO heterojunc-tion and can be used to predict accurately carrier transport and electroluminescence mechanisms forn-ZnO/MgO/p-NiO and p-NiO/MgO/n-GaN heterojunction light emitting diodes.